Important Notice

The pages on this site contain documentation for very old MS-DOS software, purely for historical purposes. If you're looking for up-to-date documentation, particularly for programming, you should not rely on the information found here, as it will be woefully out of date.

   ◄Key to Encodings►  ◄Reg►  ◄Mod►  ◄R/m►  ◄Back►
──────────────────────────────────────────────────────────────────────────────
 
  d      Direction bit. 1=register is destination, 0=register is source.
 
  w      Word/byte bit. 1=word operation, 0=byte operation.
 
  s      Sign bit. If set, sign-extend 8-bit immediate data to 16 bits.
 
  disp   Displacement. These bytes give the offset for memory operands.
         The possible lengths (in bytes) are shown in parentheses.
 
  data   Data. These bytes give the actual value for constant values.
         The possible lengths (in bytes) are shown in parentheses.
 
  sreg   Segment register. This field specifies one of the segment
         registers. 001=CS, 010=SS, 011=DS, 000=ES
 
  The mod, reg, and r/m fields are all explained on separate screens (see
  buttons at top). Reg indicates a register, while mod and r/m determine
  addressing mode. R/m can also indicate a second register.
 
  If a memory operand has a segment override, the entire instruction has
  one of the following bytes as a prefix:
 
  CS        00101110   (2Eh)
  DS        00111110   (3Eh)
  ES        00100110   (26h)
  SS        00110110   (36h)
                                    -♦-